1. Field of the Invention
The present invention relates generally to mobile communications, and in particular to methods for generating a transmission slot having at least one orthogonal frequency-division multiplexing (OFDM) symbol having variable duration.
2. Discussion of the Related Art
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as code division multiple access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously.
The current standard for CDMA systems is contained in specifications published by the Telecommunications Industry Association and Electronics Industry Association (TIA/EIA), and include IS-95A, IS-95B, and other CDMA-type protocols. New standards for wideband CDMA have been developed and offer significant performance improvements compared to the current CDMA standard. One such standard is known as cdma2000. cdma2000 is a wideband, spread-spectrum radio interface that uses CDMA technology to satisfy the needs of third generation wireless communication systems. Several enhancements of the cdma2000 standard have been developed to facilitate the gradual evolution of third generation wireless communication systems. The cdma2000 variant known as 1xEV-DO is being developed to provide high-speed packet data services as an overlay to existing circuit-switched networks.
OFDM is a technique that effectively partitions the overall system bandwidth into multiple orthogonal subbands. These subbands are also referred to as tones, carriers, subcarriers, bins, and frequency channels. Each subband is associated with a subcarrier that can be modulated with data. In each time interval that is dependent on the bandwidth of each subband, a modulation symbol may be transmitted on each of the N subbands. OFDM may be used to combat inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. ISI is caused by frequency selective fading in a multipath channel. To combat ISI, a portion of each OFDM symbol is repeated prior to transmission, as is known in the art.
Current standards typically utilize fixed-duration OFDM symbols in transmission slots communicated to mobile terminals, for example, operating within the coverage area of a communications network. Fixed-duration OFDM symbols are limiting to the extent that system transmissions cannot easily adapt to varying channel conditions experienced by the mobile terminals.